What size home battery do I need?

Multiply the backup load you want to carry by the hours you want to carry it, then divide by round-trip efficiency. A 2 kW load for 12 hours at 90% efficiency needs 26.7 usable kilowatt-hours. Add your daily peak import separately if you also want to shift consumption off expensive hours.

Why do 1 kW for 24 hours and 3 kW for 8 hours give the same answer?

Because only the product of load and hours determines energy. Both are 24 kilowatt-hours of delivered energy and both need 26.7 kWh of usable storage at 90% efficiency. Protecting fewer circuits extends the hours without buying more storage.

What is round-trip efficiency and why does it increase the requirement?

Round-trip efficiency is the share of energy put into a battery that comes back out; the rest is lost as heat. At 90% you need 26.7 kWh of storage to deliver 24 kWh. The datasheet gives the figure for your product.

Is usable capacity the same as the number on the box?

No. Manufacturers publish a nameplate capacity and a usable capacity and they differ, because a battery is not run to absolute empty. This tool returns usable kilowatt-hours, so compare it against the usable figure on the datasheet or you will undersize.

Will a big enough battery start my air conditioner or well pump?

Not necessarily, and this calculator cannot tell you. Starting a motor needs surge power, a separate specification from energy. We found one datasheet stating 185 LRA, another "up to 48 A LRA" and a third nothing. Ask for the figure in writing and check it against your largest motor.

How long will a battery run my house?

No manufacturer publishes a run time, because it depends entirely on what you connect. What we can tell you from the datasheets is that usable capacity divided by a product’s own maximum continuous output comes to roughly 1.2 to 1.3 hours across the products we read. That is the floor, at full output.

Can one battery do backup and bill shifting at the same time?

Only by splitting its capacity. Energy held in reserve for an outage is not available to shift, which is why this tool adds the two requirements rather than taking whichever is larger. If you want both fully, you need both amounts.

Should I size for a multi-day outage?

Check your utility’s actual reliability first. Sizing for a rare event is expensive, and the alternative for very long outages is a generator, which brings its own risks: the federal record includes at least 1,332 carbon monoxide deaths between 2004 and 2021. Our comparison page covers that trade-off.

Does the code limit where I can put a battery?

Yes, substantially. Residential storage must be listed and labeled to a storage standard, individual units must be separated by not less than three feet unless the listing says otherwise, there are clearances from doors and windows, and enclosing walls and ceilings carry construction requirements. Settle the location before the capacity.

Is a battery worth it financially?

That depends on your export credit more than on the battery. Under full retail net metering, storing a kilowatt-hour and exporting it are worth the same, so shifting earns nothing. Under net billing there is a gap, and our net billing calculator puts a ceiling on it to compare with the quote.

HyreSolar tools

Home battery sizing calculator

Usable kilowatt-hours for backup, for shifting peak-hour use, or for both, after charging losses. It checks energy only; the surge check is separate.

Checked September 3, 2026. Data as of August 23, 2026. HyreSolar sells and installs nothing.

This solar battery size calculator gives an estimate from your own numbers in under a minute, with every assumption listed under the result.

A home battery needs the backup load times the hours, divided by round-trip efficiency, in usable kilowatt-hours. Carrying 2 kW for 12 hours at 90% efficiency needs 26.7 kWh. Adding 8 kWh of evening peak shifting takes the total to 35.6 kWh.

Battery size depends on what you want the battery to do, and there are two different jobs. Backup means carrying chosen circuits through an outage: the energy needed is the load times the hours.

Bill shifting means storing daytime solar and using it in the evening, which pays under net billing or time-of-use rates. Many owners want both, and the calculator adds them, because a battery cannot spend the same kilowatt-hour twice.

Usable capacity is what counts, and round-trip efficiency is part of the sum: energy goes in and less comes out. The default is 90%, an allowance rather than a figure for any product; use the datasheet value for a battery you are considering.

Estimate backup load from the circuits you would run in an outage, not the whole house. A refrigerator, lights and internet need far less than central air or electric heat, and the difference decides between one battery and three.

Energy is half the check. The battery’s continuous and surge power rating, in kilowatts, must also carry your largest loads starting together, which this tool does not test.

For how long a given battery will last, use the backup runtime calculator; for what storage earns under your export tariff, use the net billing calculator.

When you have a size, the form under the result sends it to an installer for a quote on equipment that meets both checks.

Size your battery

Enter the circuits’ load and the hours you want to carry it. Nothing is stored or emailed.

Example values The filled-in numbers are an example, not your home. Type over them with your own.

— Usable capacity needed

— For backup

— For bill shifting

How this result was worked out

Working it out from the numbers above.

Energy only. Surge rating is a separate check this model does not make.

HyreSolar does not sell batteries. Compare against usable capacity, not nameplate.

Usable kilowatt-hours is not the number on the box

The battery sizing calculator returns usable energy, which is what you need to compare against a product’s usable capacity rather than its nameplate. Manufacturers publish both and they are not the same. Compare like with like.

Energy is only half the specification, and this tool models the other half not at all. A battery has an energy rating in kilowatt-hours and a power rating in kilowatts, and the power rating is what decides whether it can start your loads.

Our research found manufacturers publishing very different surge figures, 185 LRA on one product, "up to 48 A LRA" on another, and nothing at all on a third. The exclusion that bites in a real outage is surge, not energy.

Sizing for a very long outage gets expensive quickly, and there is a floor on run time nobody publishes. Across the products we read, usable capacity divided by the manufacturer’s own maximum continuous output comes to roughly 1.2 to 1.3 hours.

That is what a battery does at full output; carrying a small load for a long time is the other end of the same trade.

Under 15 kWh: A short outage or a small protected load. Realistic for a fridge, lights, internet and a few outlets.
15 to 30 kWh: Enough for an overnight outage on a well-chosen critical load panel.
Over 30 kWh: Multiple units. Check the unit-to-unit separation the residential code requires and where they are permitted to be installed.

Where to find each number

Where each input comes from, so nothing is a guess.

1. Decide what you are backing up

Not the house: a list of circuits. The list is settled at quote time and then physically wired, so it is the input to get right first.

Where to find it: Walk the house and write down what must run: refrigeration, some lighting, internet, a well pump, medical equipment, heating controls. Our critical load panel page explains why circuits move, not appliances.

2. Add up that load in kilowatts

The continuous draw of everything on the protected list running together, not the sum of every nameplate in the house.

Where to find it: Appliance labels give watts. A fridge cycles rather than running constantly, so its average is well below its label. Overstating this input buys a bigger battery than the protected circuits need.

3. Set the hours you want to carry it

How long an outage you are sizing for. Only the product of load and hours matters, so halving the load doubles the hours.

Where to find it: Your own experience and your utility’s reliability record. Our outage statistics by state give the measured picture rather than an impression.

4. Add daily peak import if you want bill shifting

The kilowatt-hours you buy during expensive hours and could instead supply from storage charged earlier.

Where to find it: A time-of-use bill shows consumption by period. If you are not on a time-varying tariff, leave this at zero: there is nothing to shift.

5. Set round-trip efficiency

The share of energy put in that comes back out. Ninety percent is a reasonable planning figure; the datasheet gives the real one.

Where to find it: The manufacturer’s datasheet states round-trip efficiency. Moving from 80% to 95% changes the requirement here from 30.0 kWh to 25.3.

How the calculator works it out

1
Backup energy
Backup load in kW × hours, divided by round-trip efficiency.
2
Bill-shift energy
Daily peak import in kWh, divided by round-trip efficiency.
3
Combine if you want both
The two add, because a battery cannot hold the same kilowatt-hour in reserve and spend it on arbitrage.
4
Then check power separately
This tool does not do it. Compare your largest starting load against the product’s surge rating.

The formula, in full. backup usable kWh = (backup load kW × hours) ÷ round-trip efficiency. shift usable kWh = daily peak import kWh ÷ round-trip efficiency. both = backup + shift.

A worked example, start to finish

A household protecting refrigeration, lighting, internet, a gas furnace’s controls and a few outlets, around 2 kW continuous, through an overnight outage, on a time-of-use tariff with 8 kWh of evening peak import.

Step	Value
Backup load	2 kW continuous
Hours to carry	12
Raw backup energy	24 kWh
Round-trip efficiency	90%
Backup requirement	26.7 kWh usable
Daily peak import	8 kWh
Shift requirement	8.9 kWh usable
Result	35.6 kWh usable

For both jobs at once. If you only want backup, 26.7 kWh. Note the losses: 24 kWh of delivered energy needs 26.7 kWh of storage, and 8 kWh of shifted consumption needs 8.9. Round-trip efficiency is a real 11% tax on everything a battery does.

How the answer moves

One input changed at a time. Every figure is computed by the calculator on this page.

Situation	Backup need	Shift need	Both
Base case2 kW × 12 h, 8 kWh shift, 90% RTE	26.7 kWh	8.9 kWh	35.6 kWh
1 kW for 24 hours	26.7 kWh	8.9 kWh	35.6 kWh
3 kW for 8 hours	26.7 kWh	8.9 kWh	35.6 kWh
Efficient battery, 95% RTE	25.3 kWh	8.4 kWh	33.7 kWh
Less efficient, 80% RTE	30.0 kWh	10.0 kWh	40.0 kWh

The first three rows are identical, and that is the lesson: only the product of load and hours matters. Halving what you protect doubles how long you can protect it, which is a far cheaper lever than buying more storage.

What moves this number most

Ranked, largest first. A proposal can change any of these without saying anything untrue, so check these inputs first.

1. What you choose to back up

The dominant input and the one entirely within your control. Cutting the protected load from 3 kW to 1.5 kW doubles your runtime for free. This decision is made when the quote is drawn and then physically wired, so it is hard to change later.

2. How long an outage you size for

Linear with energy. Twelve hours costs twice what six does. Check your utility’s actual reliability before sizing for a multi-day event.

3. Round-trip efficiency

A quiet 11% tax at the 90% default (what round-trip efficiency measures). Between 80% and 95% the requirement moves from 30.0 kWh to 25.3, which is close to a whole extra unit on some products.

4. Surge rating, which this model ignores entirely

The specification that determines whether a well pump or an air conditioner will start. Manufacturers publish it inconsistently: we found 185 LRA stated on one product, "up to 48 A LRA" on another, and no figure at all on a third.

5. Where the battery is allowed to go

The residential code restricts placement, requires not less than three feet between units, sets clearances from doors and windows and imposes construction requirements on the enclosing space. Establish location before capacity.

6. Whether the utility gives you a reason to shift

The bill-shift half of this calculation is worth nothing on a flat tariff. Check the gap between your import rate and export credit before paying for capacity to arbitrage it.

Common mistakes with this calculation

Sizing for the whole house
A whole-home backup is a much larger and more expensive proposition than a list of critical circuits, and at least one manufacturer guide forbids it outright for its product. Back up circuits, not a building.
Adding up nameplate ratings instead of realistic loads
A refrigerator cycles rather than running continuously, so its average draw is far below its label. Summing labels produces a battery specification nobody needs.
Comparing usable kWh against nameplate kWh
Manufacturers publish both and they differ. This tool returns usable energy, so compare it against a product’s usable figure or you will undersize.
Ignoring the power rating
Energy tells you how long; power tells you whether it starts at all. A battery with ample kilowatt-hours can still fail to start a well pump. Ask for the surge rating in writing and check it against your largest motor load.
Assuming you can have backup and arbitrage from the same kilowatt-hours
You cannot. Energy held in reserve for an outage is energy not available to shift, which is why this tool adds the two requirements rather than taking the larger.
Deciding capacity before deciding location
The code restricts where storage may go and requires separation between units. A capacity that needs three units may not have anywhere compliant to put them.

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What this calculator leaves out

The result is a planning estimate. A real proposal accounts for these; this page does not.

Energy only.
This model says nothing about the power or surge rating, which is what decides whether your loads will start.
Returns usable kilowatt-hours.
Compare against a product’s usable capacity, not its nameplate.
Assumes the battery starts full and that you can choose what it powers.
Does not model depth of discharge limits, reserve settings, degradation over time or temperature derating.
Does not check where the code permits the storage to be installed, or how many units fit there.

The research behind these numbers

The critical load panelWhich circuits actually run in an outage, why circuits move rather than appliances, and the code requirements on where storage goes.Read the research Battery backup versus a generatorWhy surge and not energy is the exclusion that bites, and the federal carbon monoxide record of the alternative.Read the research Home battery chemistryWhat manufacturers do and do not publish. One major product has no chemistry row on its datasheet at all.Read the research Power outage statistics by stateHow long outages actually last where you live, computed from federal data rather than impression.Read the research Solar battery statisticsHow many net-metered systems pair storage, by state, from EIA-861.Read the research Off-grid versus grid-tiedWhy a grid-tied system produces nothing in an outage without storage, and what the listing requirement actually says.Read the research

Questions this calculator answers

What size home battery do I need?: Multiply the backup load you want to carry by the hours you want to carry it, then divide by round-trip efficiency. A 2 kW load for 12 hours at 90% efficiency needs 26.7 usable kilowatt-hours. Add your daily peak import separately if you also want to shift consumption off expensive hours.
Why do 1 kW for 24 hours and 3 kW for 8 hours give the same answer?: Because only the product of load and hours determines energy. Both are 24 kilowatt-hours of delivered energy and both need 26.7 kWh of usable storage at 90% efficiency. Protecting fewer circuits extends the hours without buying more storage.
What is round-trip efficiency and why does it increase the requirement?: Round-trip efficiency is the share of energy put into a battery that comes back out; the rest is lost as heat. At 90% you need 26.7 kWh of storage to deliver 24 kWh. The datasheet gives the figure for your product.
Is usable capacity the same as the number on the box?: No. Manufacturers publish a nameplate capacity and a usable capacity and they differ, because a battery is not run to absolute empty. This tool returns usable kilowatt-hours, so compare it against the usable figure on the datasheet or you will undersize.
Will a big enough battery start my air conditioner or well pump?: Not necessarily, and this calculator cannot tell you. Starting a motor needs surge power, a separate specification from energy. We found one datasheet stating 185 LRA, another "up to 48 A LRA" and a third nothing. Ask for the figure in writing and check it against your largest motor.
How long will a battery run my house?: No manufacturer publishes a run time, because it depends entirely on what you connect. What we can tell you from the datasheets is that usable capacity divided by a product’s own maximum continuous output comes to roughly 1.2 to 1.3 hours across the products we read. That is the floor, at full output.
Can one battery do backup and bill shifting at the same time?: Only by splitting its capacity. Energy held in reserve for an outage is not available to shift, which is why this tool adds the two requirements rather than taking whichever is larger. If you want both fully, you need both amounts.
Should I size for a multi-day outage?: Check your utility’s actual reliability first. Sizing for a rare event is expensive, and the alternative for very long outages is a generator, which brings its own risks: the federal record includes at least 1,332 carbon monoxide deaths between 2004 and 2021. Our comparison page covers that trade-off.
Does the code limit where I can put a battery?: Yes, substantially. Residential storage must be listed and labeled to a storage standard, individual units must be separated by not less than three feet unless the listing says otherwise, there are clearances from doors and windows, and enclosing walls and ceilings carry construction requirements. Settle the location before the capacity.
Is a battery worth it financially?: That depends on your export credit more than on the battery. Under full retail net metering, storing a kilowatt-hour and exporting it are worth the same, so shifting earns nothing. Under net billing there is a gap, and our net billing calculator puts a ceiling on it to compare with the quote.

Sources and methodology

Figures dated August 23, 2026. Last reviewed September 3, 2026 by the HyreSolar research desk, which writes and checks every tool on this site.

2024 International Residential Code, energy storage systems (ICC, retrieved September 3, 2026). Listing, separation, location and construction requirements referenced above.
IQ Battery 5P Data Sheet (Enphase, retrieved September 3, 2026). One of the datasheets behind the surge-rating and usable-capacity observations.
Form EIA-861, Net Metering (EIA, retrieved August 23, 2026). Underlying data for our storage attachment analysis.

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